1. Technical Field
The disclosed subject matter relates to a light source unit, and more particularly to a light source unit for reading objects in a reader, such as a scanner.
2. Description of the Related Art
In a conventional process for reading a document in a scanner with a shrinkage optical system or a contact optical system, a line-shaped scanning light having a certain width may be emitted or applied onto the document image. The reflected light or transmitted light is then image-formed on a line solid-state imaging device (line CCD) through an optical path including mirrors and lenses. The output signal from the line solid-state imaging device (line CCD) is processed to read the document image.
Among conventional devices, there has been proposed an irradiation optical system configured as shown in FIG. 7 to apply light emitted from a light source to a document image.
The system comprises a carriage 50, which houses a line light source 51 such as a halogen lamp and a reflector 52 arranged in the vicinity of the line light source 51. Direct light from the light source 51 and light emitted from the light source 51 and reflected at the reflector 52 are collected and, after transmission through a contact glass 53, applied to or incident on a document image 54. The light reflected at the document image 54 and transmitted through the contact glass 53 is guided via a mirror 55 similarly housed in the carriage 50 to an optical path reaching a line CCD (for example, see Japanese Patent document 11-27458A).
In the document image reading method configured above, the width of the line-shaped light that is applied onto the document image, falling within 20% the fluctuation of the amount of light, is determined to be 1.5 times or more the width of the image forming area in the line CCD. In this case, an error in accuracy of attachment of the optical moving part (carriage) and a displacement of the irradiated position due to vibrations when moving (scanning) may occur. Even in such a situation, it is possible to suppress the fluctuation of the amount of light on the document image within the image forming area in the line CCD.
In the above-described document image reading method, the light emitted from the light source is split into two optical paths: a first optical path and a second optical path. On the first optical path, the light transmits through the contact glass 53 directly and reaches the document image 54. On the second optical path, the light is reflected from the reflector 52 and the reflected light transmits through the contact glass 53 and reaches the document image 54.
In this case, a wide-directivity halogen lamp is used as the light source 51, and the reflector 52 is located in the vicinity of the light source 51 to improve the effect of collecting light. The amount of light traveling on the second optical path to the document image 54 is, however, less than the amount of light traveling on the first optical path to the document image 54.
The distribution of intensities of illumination in the direction of movement (sub-scan direction) in a width dimension of the line-shaped light that is applied onto the document image 54 therefore becomes uneven and causes variations in reading the document image 54.
The wide-directivity light source provides a large amount of light that can not contribute to reading the document image 54 ( i.e., there is an inherent loss in light) and the light from the light source can not be utilized sufficiently. The use of LED light sources having an extremely lower amount of total light beams as compared to the halogen lamp to achieve an amount of illuminating light equal to the halogen lamp requires a large number of LED light sources.
To solve such a problem, two halogen lamps may be provided in parallel beneath the contact glass to apply symmetrical light sources from both the halogen lamps to a document image above the contact glass, thereby achieving an excellent distribution of intensities of illumination with the illuminating light.
In this case, an optically excellent distribution of intensities of illumination can be obtained. However, the use of two halogen lamps in the light source increases the material expense and size and causes an increase in production costs.
The disclosed subject matter has been made in consideration of the above and other problems and can be configured to provide a light source unit for reading documents at low cost, while excellent in utilization of light, with a higher intensity of illumination of document images, and being capable of applying a light with an excellent distribution of intensities of illumination while taking up a small amount of space.